1. Field of the Invention
This invention relates to surgical fasteners for fastening body tissue and more particularly relates to an at least partially absorbable fastener for fastening bone or hard tissue.
2. Background of the Related Art
Bone fastening or fixation devices are well known in the art. Typically, such fasteners are in the form of staples, pins, screws, and wires. For example, both Pratt et al., U.S. Pat. No. 4,454,875 and Ellison et al., U.S. Pat. No. 4,570,623 disclose staples for being driven into bones. Staples of this type are generally fabricated of biologically inert metal, such as stainless steel, titanium, cobalt-chromium-molybdenum alloys and the like. The staple must be relatively strong and hard so that it can be easily driven directly into bone or hard tissue.
Other metal fasteners are well known commercial products used for a wide variety of bone fixation procedures. Pins and wires are generally constructed from stainless steel and are grasped in a drill chuck and self-drilled directly into bone to treat a given traumatic or pathological condition.
The disadvantage of metal fasteners is that after they have completed their function of supporting the bone while the bone heals, they remain permanently in the body. Problems can arise after healing, for example, by corrosion of the metal, or when the pins or staples work loose from their moorings and migrate through body tissue.
Furthermore, permanent metal fixation devices shield the bone from beneficial stresses after healing. It has been shown that moderate periodic stress on bone tissue, such as the stress produced by exercise, helps to prevent decalcification of the bone. Under some conditions, the stress shielding which results from the long term use of metal bone fixation devices can lead to osteoporosis.
These disadvantages can be mitigated by the use of bioabsorbable surgical fasteners, which degrade over a period of time thereby gradually transferring more support load to the bone as it heals. Such fasteners for bone are also known in the art. For instance, Tunc, U.S. Pat. No. 4,539,981 teaches the use of polymers of L(-)lactide for fabricating bone fixation devices. Moreover, various types of bioabsorbable pin fasteners have been commercialized. For example, some types of pins are fabricated from poly (p-dioxanone) and are indicated for use to fix in place small bony fragments in the knee and hand, where such fragments are not in tension. As is characteristic for all such absorbable pins, holes must be previously drilled into the bone in order for the pins to be inserted. Bioabsorbable fasteners are not self-inserting, i.e. they are not capable of being driven or screwed directly into bone because the polymeric material they are made of is relatively soft. The necessity to predrill holes in the injured bone adds to the surgical procedures and lengthens the time required to complete the operation.
Up to now, there has been no surgical bone fastener which retained the advantages of the above mentioned types of fasteners, without the concurrent disadvantages.